Add alerts to Celo alert bot

[cfl0ws]

Proposal

• Alert when new proposal is submitted and enters voting stage

• Include voting stage deadline

Approval

• Alert when proposal enters approval stage.

• Include approval stage deadline

• Alert when new proposal has been approved or expired

Referendum

• Alert when proposal enters referendum stage

• Include referendum (voting) stage deadline
• Include defined required level of agreement
• Include defined required agreement for a proposal

• Alert when new proposal has been approved and moves to execution phase

Execution

• Alert when proposal enters execution stage.

• Include execution

• Alert when has been executed or if proposal expires, i.e. is not executed